1. Field of the Invention
The present invention relates to a contact material for forming the contacts of a vacuum interrupter excellent in large current interruption ability, chopping current characteristics, current carrying characteristics and large current carrying characteristics, and to a method of manufacturing the contacts.
2. Description of the Related Art
A vacuum interrupter which interrupts a current by using the property of arcs to diffuse in a vacuum has two opposite, stationary and movable contacts. When interrupting a current flowing through an inductive load circuit, such as an electric motor, by the vacuum interrupter, it is possible that the load circuit is destroyed by an extraordinarily high surge voltage generated upon the interruption of the current.
Such an extraordinarily high surge voltage is generated, for example, by a current chopping phenomenon (forced current interruption before the alternating current decreases to the natural zero point of the ac waveform) that occurs when low current interruption is made in a vacuum or by a high-frequency arc extinguishing phenomenon. A surge voltage V.sub.s generated by a current chopping phenomenon is equal to Z.sub.o.times.I.sub.c, where Z.sub.o is the impedance of the circuit and I.sub.c is a chopping current. Therefore, the chopping current I.sub.c must be reduced to reduce the abnormal surge voltage V.sub.s.
Contacts having a low chopping current characteristics are classified into a Cu--Bi alloy contacts formed by a melting process, and an Ag--WC alloy contacts formed by a sintering-and-infiltration process.
The Ag--WC alloy contact exhibits an excellent low chopping current characteristics in the following respects:
(1) WC facilitates electron emission; PA1 (2) Evaporation of the material forming the contact due to the heating of the surface of the contact by the collision of field emission electrons against the contact is promoted; and PA1 (3) The carbide contained in the material forming the contact is decomposed by arcs to generate charged bodies and the charged bodies make the arc continuous. A vacuum circuit interrupter provided with such alloy contacts has recently been developed and applied to practical use.
An Ag--Cu--WC alloy produced by adding Cu to the Ag--WC alloy forming the Ag--WC alloy contact in an Ag/Cu ratio of about 7/3 is proposed in Japanese patent publication JP B2(kokoku) No. S63-59212. This known Ag--Cu--WC alloy having such a specific Ag/Cu ratio has a stable chopping current characteristics.
A technique disclosed in Japanese patent publication JP B2(kokoku) No. H05-61338 suggests that the use of an arc-proof material, such as a WC alloy having a particle size in the range of 0.2 to 1 .mu.m is effective in improving chopping current characteristics.
The chopping current characteristics of the Cu--Bi alloy contact is improved by the selective evaporation of Bi. A Cu--Bi alloy having a Bi content of 10 percent by weight (hereinafter abbreviated to "% by weight" or "wt %") proposed in Japanese patent publication JP B2(kokoku) No. S35-14974 has a moderate vapor pressure characteristic and hence exhibits a low chopping current characteristics. In a Cu--Bi alloy having a Bi content of 0.5% by weight proposed in Japanese patent publication JP B2(kokoku) No. S41-12131, Bi segregates in grain boundaries, which embrittles the Cu--Bi alloy. Therefore, a contact formed of this Cu--Bi alloy needs a low weld release force and has an excellent large current interruption ability.
Essentially, the vacuum interrupter must be capable of large current interruption. It is important to produce arcs over the entire surfaces of the contacts to limit the amount of input heat per unit area of the contacts to a low level to enable the contacts to achieve large current interruption. One of the means for producing arcs over the entire surfaces of the contacts employs a longitudinal magnetic field electrode structure which creates a magnetic field parallel to an electric field created between electrodes mounted with contacts. According to Japanese patent publication JP B2(kokoku) No. S54-22813, an arc plasma can be distributed uniformly over the surfaces of the contacts by creating an appropriate magnetic field, and large current interruption ability can be enhanced.
According to Japanese patent publication JP A(kokai) No. H04-206121, the mobility of the cathode point of an arc is improved and large current interruption ability is improved when interparticle distances between WC--Co particles of an Ag--Cu--WC--Co alloy are in the range of about 0.3 to about 3 .mu.m, and the interrupting performance of contacts made of such an alloy is improved when auxiliary ferrous component content, such as Co content, is increased.
The vacuum interrupter is required to be capable of suppressing surges and a low chopping current characteristics has been required of the vacuum interrupter. Cases where the vacuum interrupter is applied to inductive circuits, such as large capacity electric motors, have increased in recent years, and high surge impedance loads have appeared. Consequently, it is desired that the vacuum interrupter has both a further stable low chopping current characteristics and a large current interruption ability.
When contacts made of the Cu--Bi alloy having a Bi content of 10% by weight mentioned in JP B2(kokoku) No. S35-14974 are used, the amount of metal vapor supplied into the space between the electrodes decreases with the increase of current interrupting frequency of the contacts, the low chopping current characteristics deteriorates and deterioration of the withstand voltage characteristics dependent on the amount of an element having a high vapor pressure occurs. The low chopping current characteristics of the Cu--Bi alloy having a Bi content of 0.5% by weight (mentioned in JP B2 No. S41-12131) is unsatisfactory. Thus, it is impossible to provide a stable low chopping current characteristics only by the selective evaporation of the element having a high vapor pressure.
Although a contact material containing Ag as a conductive component, such as an Ag--WC--Co alloy, has a comparatively satisfactory chopping current characteristic, the vapor pressure of the same is excessively high and the same is unable to exercise fully satisfactory interrupting performance. Although the Ag--Cu--WC alloy having the Ag/Cu weight ratio of 7/3 (mentioned in JP B2 No. S63-59212) and a contact material containing Ag as a principal conductive component, such as the Ag--Cu--WC alloy containing particles of an arc-proof component, such as WC particles, having a particle size in the range of 0.2 to 1 .mu.m (mentioned in JP B2 No. H05-61338) are excellent in interruption ability and chopping current characteristics, contacts made of those alloys are expensive. Increase in the Co content of those contact materials to improve the interrupting performance entails the reduction of the low chopping current characteristic.
On the contrary, a contact material containing Cu, which is inexpensive, as a conductive component, is comparatively satisfactory in interrupting ability, however, the chopping current characteristics of the contact material is unsatisfactory unless the arc-proof component content is increased. For example, the porosity of a WC skeleton is reduced by adding Co to the WC skeleton when sintering the WC skeleton for the Cu--WC--Co alloy to suppress the infiltration of Cu into the WC skeleton.
However, components for promoting the sintering of carbides, such as Co, Fe and Ni, reduce the conductivity of Cu and hence the current carrying characteristic of the alloy is deteriorated greatly if the alloy contains those components excessively.